1. Field of the Invention
This invention relates to a mechanical process for reducing residual welding stresses in the weld metal, heat-affected zone (HAZ) and in the adjacent base metal of steel piping that have been butt-welded to each other end to end by means of a circumferential weld.
2. Information Disclosure Statement
When piping are butt welded together by means of circumferential weld, significant residual tensile welding stresses are produced in the weld metal, in the heat-affected zone and in the adjacent base metal of said piping. These tensile stresses tend to enhance stress corrosion cracking at the inside surface, fatigue and creep crack propagation in the weld metal, heat-affected zone and in the adjcaent base metal of such piping.
Induction heating stress improvement (IHSI) is one method currently being used to improve residual welding stresses in pipes and shells. However, the IHSI process cannot induce net compressive forces in either the axial or circumferential directions. Equilibrium of the axial compressive stresses due to the IHSI process induces axial tensile stresses such that the net axial force in every cross-section is zero. Indeed, through-the-wall bending stresses are induced during the IHSI process. Weld defects, such as inclusions, porosity, lack of fusion, hot tears, etc., and stress corrosion or fatigue cracks are therefore subjected to high tensile stresses during the process. The resulting crack opening, fatigue damage and creep crack propagation increases exposure to the corrosive media and reduces the remaining strength and endurance of the pipe or shell walls, and makes them more susceptible to further cracking and leaking.
Our Mechanical Stress Improvement Process is an improved method in that high tensile stresses are not induced in the piping during the claimed process, thus eliminating the potential deleterious effects of these tensile stresses.
Fencl in U.S. Pat. No. 4,018,634, dated Apr. 19, 1977, produces a high-strength steel pipe by forming a mother plate into the shape of a cylinder, the longitudinal edges thereof are joined by welding, so that the resulting pipe has a continuous longitudinal weld seam, shrinks the entire pipe thus formed in the radial direction by applying compressive radial pressure to the entire outer surface of the pipe, so the pipe diameter is reduced by at least 1.5 percent, after which the pipe is heated to a temperature below the transformation temperature of the steel pipe but high enough to increase the circumferential yield stress of the entire pipe. Fencl, therefore, neither discloses nor teaches, nor is related to, the process defined and claimed herein, namely, the reduction of residual tensile welding stresses solely in the weld metal, heat-affected zone and in the adjacent base metal of piping that have been butt-welded end to end by means of a circumferential weld.